This program project is aimed at characterizing the regulatory mechanisms that control the G1/S transition in mammalian cells. To this aim, the three projects require the use of: (a) tissue cultures from different origins, including mouse embryonic fibroblasts derived from genetically modified mice. (b) recombinant viruses to ectopically express particular oncogenes and potential tumor suppressor genes studied in the three projects. These services will be centralized by the Virology/Cell Culture Core, where competent experienced personnel will be in charge of maintaining stocks of all necessary cell cultures and generating different kinds of recombinant viruses according to the needs of the investigators on each project. By centralizing these services for the three projects an economy of space and resources will be achieved. Core B will be in charge of generating and maintaining primary cultures and stocks of MEFs (Projects 1, 2 and 3), tumor cell lines generated from CDK4 (R24C/R24C) mice, and the various cell lines required for the projects described in this application. In addition, Core B will generate and maintain stocks of three different kinds of replication defective recombinant viruses: adenovimses, retroviruses, and/or lentiviruses. Each type of recombinant virus will be suitable for specific experiments according to their particular characteristics in terms of cells that can infect and stability of the transduced genetic material.